19-coroutine/coroaverager3:grouper

[ramalho]

Issue originally reported by @dongzeXD at https://github.com/fluentpython/example-code-2e/issues/15, related to an example in post Classic Coroutines:

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In <6> annotations ："Each iteration in thisloop creates a new instance of averager; each is a generator object operating as a coroutine." and <7>:"Whenever grouper is sent a value, it’s piped ．．．, the value it returns is bound to results[key]. The while loop then proceeds to create another averager instance to consume more values."

Would I replace while True with yield? This seems easier to understand